Semiconductor optical amplifier

ABSTRACT

The invention concerns a semiconductor optical amplifier including at least two amplifier sections ( 30, 40 ) respectively favoring a higher gain of the TE mode and the TM mode of polarization of the light to be amplified, said sections each having an active guide structure ( 12 ) of the same thickness (e), the amplifier being characterized in that the active guide structure ( 12 ) of the two sections ( 30, 40 ) is subjected to respective different tension stresses and/or has a different geometry so as to render the overall gain of the amplifier insensitive to the polarization of said light to be amplified, and in that there is no discontinuity of the effective refractive index at the transition between the sections ( 30, 40 ).

The present invention relates to amplifying optical signals. It finds atypical application in fiber optic telecommunication networks. Thesignals transmitted by fiber optic telecommunication networks consist ofpulses carrying information to be transmitted in binary form. The pulsesmust be amplified to compensate power losses that they suffer duringtheir propagation in said networks. Semiconductor amplifiers constitutea compact means of obtaining such amplification and can be integrated.However, unless specific measures are implemented to prevent it, theirgain is sensitive to the state of polarization of the light that theyreceive, as indicated more simply hereinafter by referring to thepolarization-sensitivity of an amplifier.

The invention finds a particular application when it is necessary toeliminate or at least limit polarization-sensitivity, which can beexpressed by the following equation: ΔG=G_(TE)−G_(TM). The aim is toachieve the condition |ΔG|<1 dB.

The situation in which the sensitivity must be limited or eliminated isfrequently encountered and arises when the distance traveled by theoptical pulses to be amplified is such that the state of polarization ofthe pulses has been significantly and randomly affected during theirpropagation and it is preferable for the amplified pulses to have one ormore predetermined power levels.

More generally, the invention finds an application whenever an opticalamplifier must have no polarization-sensitivity or a lowpolarization-sensitivity.

The invention applies more specifically to buried ridge structure (BRS)amplifiers.

A buried ridge structure semiconductor optical amplifier (see FIG. 1)includes a wafer 2 made up of layers of semiconductor materials havingrespective refractive indices and forming a common crystal lattice. Inthe absence of mechanical stresses, the lattices formed by thesematerials have respective characteristic dimensions constitutingrespective lattice parameters of the materials. The layers are insuccession in a vertical direction DV forming a right-angle trihedron,defined with respect to the wafer 2, with two horizontal directionsconstituting a longitudinal direction DL and a transverse direction DT.The layers form an upward succession in the vertical direction DV from abottom face 4 to a top face 6. The wafer 2 includes at least thefollowing layers or groups of layers or parts of layers:

A substrate 8 consisting mainly of a semiconductor basic material havinga first type of conductivity. This substrate is sufficiently thick toimpose the dimensions of the lattice of the basic material on all of thecrystal lattice of the wafer 2.

An active layer 10 including an active material adapted to amplify lightby stimulated recombination of charge carriers of both types injectedinto the material.

Finally, a top confinement layer 18 consisting of a material having asecond type of conductivity which is the opposite of the first type.

The amplifier further includes a bottom electrode 20 and a top electrode22 respectively formed on the bottom face 4 and the top face 6 of thewafer 2 to enable an electrical current to flow between said faces forinjecting said charge carriers of both types into the active material.

The basic materials of prior art semiconductor optical amplifiers areIII-V materials, typically indium phosphide and gallium arsenide. Theactive material is typically a ternary or quaternary material containingthe same chemical elements. It is generally required for the width l ofthe guide active structure 12 which guides the light to be close to onemicrometer, to facilitate etching it and most importantly to facilitateintegrating the amplifier with other optical components on the samesemiconductor wafer. To ensure monomode propagation of light, typicallyat a wavelength of 1 310 nm or 1 550 nm, the thickness e must then bevery much less than the width l. If no special measures are applied toprevent it, this rectangular shape of the section of the guide activestructure 12 causes the polarization-sensitivity previously mentioned.

In BRS amplifiers, the active material 10 constituting the activestructure 12 guiding the light is surrounded on all sides by a binarysemiconductor material 14, 16. This material has the advantage ofconducting heat well, but its refractive index is only slightly lowerthan that of the active material. Consider further the situation inwhich the active material is homogeneous, in which case it is referredto as a bulk material. As a general rule, the section of the buriedguide active structure 12 is strongly rectangular. Given the small indexdifference between the guide structure 12 and the surrounding binarymaterial 14, 16, the confinement of a horizontally polarized wave isgreater than that of a vertically polarized wave, the difference betweenthe two confinement factors increasing as the ratio of the width l tothe thickness e of the guide structure 12 increases. The confinementreferred to above in connection with a wave is in a transverse plane. Itis equal to the ratio of the power of the wave in the area occupied bythe guide structure to the total power of the wave. The confinementfactor is defined for each polarization and for each wavelength by theshape and dimensions of the section of the ridge and by the refractiveindices of the material of the ridge and the surrounding material. Inthe case of a rectangular section, it can be considered to be theproduct of a directional confinement factor in the horizontal directionby a directional confinement factor in the vertical direction, each ofthe two directional confinement factors depending on the polarization.Given that the phenomenon of amplification of the wave by recombinationof carriers and stimulated emission occurs only in the active material,i.e. in the ridge 12, the gain of the amplifier for a wave increases asthe confinement of the wave increases. As a result of this, if thematerial of the guide structure 12 were a homogeneous material, and alsoisotropic, and therefore insensitive to polarization, the gain of theamplifier would be greater for horizontally polarized waves than forvertically polarized waves.

Considerable research has been conducted into making these amplifiersinsensitive to the polarization of the light to be amplified.

In particular, the applicant's U.S. Pat. No. 5,982,531 proposes anamplifier of this kind that is rendered insensitive to the polarizationof the light. The amplifier is characterized in that its active materialis subjected to a sufficient tension stress to render its gaininsensitive to the polarization of said light to be amplified. Thisstress generally results from a lattice mismatch between the activematerial and the basic material of the substrate. The horizontalconfinement factor is typically equal to the product of the verticalconfinement factor by a confinement asymmetry coefficient.

The present patent application is based on the observation that, even inthe presence of a high confinement asymmetry coefficient resulting, forexample, from the fact that the guide structure 12 consists of a ridgeof strongly rectangular section, the tension stress to be applied to ahomogeneous active material forming said ridge to obtain insensitivityto polarization is sufficiently low for the thickness of the ridge toremain less than the corresponding critical thickness relating todislocations.

The above kind of amplifier has a low sensitivity to polarization. Itsmain parameters are:

the wavelength of the amplifying active layer: λ=1.57 μm,

the active material: In_(1−x)Ga_(x)P_(1−y)As_(y),

the tension stress of the active layer: Δa/a=−0.015,

the thickness of the active layer: e=0.2 μm, and

the width of the ridge: l=1 μm.

This kind of structure has drawbacks, however. It has been establishedexperimentally and theoretically that the polarization depends stronglyon the control of the thickness of the active layer and the stresses towhich it is subjected. For example, modifying this stress (Δa/a) from−0.015 to −0.014 or −0.016 induces a gain shift ΔG of 0.8 dB toward arespective sensitivity of the TE mode or the TM mode. Similarly, aslight modification of the thickness of the active layer (of a fewpercent) directly induces a gain offset ΔG of the amplifier. Accordinglythe sensitivity of the amplifier to the polarization of the lightdepends on its structure and cannot be controlled easily.

The object of the present invention is to remove the drawbacks of thetechnology proposed by the previously cited U.S. Pat. No. 5,982,531.

To this end, the present invention proposes a structure such that thepolarization-sensitivity of the overall gain ΔG of the amplifier iseasily controlled by a current, to provide “active” adjustment ofpolarization-sensitivity.

The semiconductor optical amplifier according to the invention thereforehas at least two separate sections, each provided with an electrode andeach having a different geometry and/or tension stress so asrespectively to favor a higher gain of the TE mode and the TM mode.

A structure of this kind with two sections has already been disclosed,in particular in Japanese Patent Application JP 10154841, whichdiscusses a solution consisting of varying the thickness of the activelayer from one section to the other to influence the gain byrespectively favoring the TE mode with a small thickness and then the TMmode with a greater thickness.

To obtain a polarization-independent amplifier the gains of the twosections can be adjusted by varying the current injected into eachelectrode of each section.

However, the solution proposed in the above Japanese patent applicationhas disadvantages, in particular from the point of view of its technicalimplementation.

On the one hand, the transition between the two sections is abrupt,which causes a non-adiabatic modification of the sizes of the modespropagating in the active layer, and causes a reflection of the lightwaves at the transition. Reflections in a SOA are unacceptable.

On the other hand, implementing this kind of structure necessitates astep of etching the active layer, which must be perfectly controlled,followed by a step of further epitaxial growth. Well-controlled etchingnecessitates dry etching followed by chemical etching. This kind oftechnique is generally avoided on active materials because it inducessurface recombination effects which degrade the quality of the activelayer. Also, the further epitaxial growth step is particularly difficulton a thin active layer.

The present invention aims to remove these drawbacks by proposinganother structure with two sections enabling “active” adjustment byrespectively favoring a higher gain of the TE mode and the TM mode.

The structure proposed by the invention entails producing two sectionshaving an active layer of the same thickness but subjected to differenttension stresses and/or having different geometries, whilst at the sametime ensuring continuity of the effective refractive index of the activelayer in the two sections for a transition that is adiabatic or with noindex step.

To be more specific, the present invention consists in a semiconductoroptical amplifier including at least two amplifier sections respectivelyfavoring a higher gain of the TE mode and the TM mode of polarization ofthe light to be amplified, said sections each having an active guidestructure of the same thickness, the amplifier being characterized inthat the active guide structure of the two sections is subjected torespective different tension stresses and/or has a different geometry soas to render the overall gain of the amplifier insensitive to thepolarization of said light to be amplified, and in that there is nodiscontinuity of the effective refractive index at the transitionbetween the sections.

In a first embodiment the active guide structure has a respectivedifferent width in the sections.

In a second embodiment the active guide structure of at least onesection has a curved portion.

In a third embodiment the active guide structure of the sections issubjected to respective different tension stresses.

In accordance with one feature of the third embodiment the active guidestructure of the sections is made from a material having differentstoichiometric ratios between the elements constituting said material.

In accordance with one feature of the invention the material of theactive guide structures is a quaternary material.

In accordance with one feature of the invention the quaternary materialis InGaAsP.

The features and advantages of the invention will become clearlyapparent on reading the following description, which is given by way ofnon-limiting illustrative example and with reference to the accompanyingdrawings, in which:

FIG. 1, already described, is a diagram of a prior art buried ridgestructure semiconductor optical amplifier;

FIG. 2 is a diagrammatic plan view of a first embodiment of an amplifieraccording to the invention;

FIG. 3 is a diagrammatic plan view of a second embodiment of anamplifier according to the invention;

FIG. 4 is a diagrammatic plan view of a third embodiment of an amplifieraccording to the invention.

The invention provides a semiconductor optical amplifier whose gain isindependent of the polarization of the light to be amplified.

The amplifier according to the invention has two amplifying sections 30and 40 respectively favoring a higher gain of the TE mode and the TMmode of polarization of the light to be amplified, and each section 30and 40 is controlled by a respective separate electrode 23 and 24.

Accordingly, by “active” adjustment, obtained by controlling the currentinjected into each electrode 23 and 24, it is possible to favor one orthe other mode of polarization of the light to be amplified, to renderthe overall gain of the amplifier polarization-insensitive. The currentapplied must be sufficiently high to avoid problems due to noise, butwithout being too high, which would reduce the effects of electricalcontrol of the polarization of the light.

According to one feature of the invention, the amplifier includes asingle active guide structure 12 consisting of an etched and buriedridge. The ridge 12 is common to the sections 30 and 40 and has the samethickness everywhere.

The material of the active guide structure is preferably a quaternarymaterial, for example InGaAsP.

The active guide structure 12 nevertheless has properties specific toeach section 30 and 40 to favor a respective mode of polarization of thelight to be amplified.

In a first embodiment, shown in FIG. 2, the active guide structure 12has a different width l₁, l₂ in each section 30 and 40. The confinementfactor of the wider ridge portion favors the TE propagation mode and theconfinement factor of the narrower ridge portion favors the TMpropagation mode.

This kind of ridge 12 is easy to produce by etching with a suitable maskthat defines the respective widths of each section 30 and 40.

In various embodiments, the width l₁ of the active guide structure 12 ofthe section 30 favoring a higher gain of the TE mode is from 0.8 to 1.2μm and the width l₂ of the active guide structure 12 of the section 40favoring a higher gain of the TM mode is from 0.6 to 1.0 μm, with thecondition l₁>l₂ still met.

Unlike a difference in thickness, this difference in the width of theactive ridge 12 ensures an adiabatic mode transition between the twosections 30 and 40, which eliminates the risk of reflection of lightwaves.

In a second embodiment, shown in FIG. 3, the active guide structure 12has a curved portion 13 in the section 30 favoring the TE mode ofpropagation of the light to be amplified.

As previously, the material of the active guide structure and theconfinement factor are the same in both sections 30 and 40. The curvedsection 13 of the ridge 12 favors the TE mode of propagation of lightand the straight sections favor the TM mode (because of the nature ofthe material constituting the ridge). The straight ridge sections 40 areseparated by the curved section 13 in the example shown, but areelectrically connected by interconnected electrodes 24, 24′.

This kind of ridge 12 with a curved portion 13 is easily produced byetching with a suitable mask. Once again, an adiabatic mode transitionis obtained between the two sections 30 and 40, which eliminates therisk of reflection of light waves.

In a third embodiment, shown in FIG. 4, the active guide structure 12 issubjected to respective different tension stresses in the sections 30and 40.

The active guide structure 12 is made from a quaternary material. Thetension stress difference between the two sections 30 and 40 is obtainedby a difference between the stoichiometric ratios of the elementsconstituting the material of said active structure 12.

The use of the same material (InGaAsP) avoids an index step on passingfrom one section to the other and consequently reflection of light wavesbetween the sections 30 and 40. It is the composition of the materialthat varies.

Accordingly, for a given wavelength λ (for example 1.5 μm) of the lightto be amplified, several pairs of values (x, y) are available forapplying different lattice tensions to the In_(1−x)Ga_(x)As_(1−y)P_(y)material in each section.

The ridge 12 in each section is made by double epitaxial growth usingthe proven prior art butt-coupling technique.

The three embodiments described are not limiting on the presentinvention, and in particular can be combined with each other withoutdeparting from the scope of the invention.

What is claimed is:
 1. A semiconductor optical amplifier including atleast two amplifier sections (30, 40), a first section favoring a highergain of the TE mode and a second section favoring a higher gain of theTM mode of polarization of the light to be amplified, said sections eachhaving an active guide structure (12) constituted by a solid materialand of identical thickness (e), the amplifier being characterized inthat each section (30, 40) of the active guide structure (12) issubjected to different tension stresses so as to render the overall gainof the amplifier insensitive to the polarization of said light to beamplified, and in that there is no discontinuity of the effectiverefractive index at the transition between said sections (30, 40).
 2. Asemiconductor optical amplifier according to claim 1, characterized inthat the active guide structure (12) of the sections (30, 40) is madefrom a material having different stoichiometric ratios between theelements constituting said material.
 3. A semiconductor opticalamplifier according to claim 1, characterized in that the material ofthe active guide structures (12) is a quaternary material.
 4. Asemiconductor optical amplifier according to claim 3, characterized inthat the quaternary material is InGaAsP.